SECRETION 1 . 63 



epithelium, with tlie cells of a columnar form, is found cloth- 

 ing the respiratory passages, including the lower part of 

 the nasal cavity, the larynx, trachea, and bronchial tubes ; 

 it also lines the Eustachian tubes, and parts of the repro- 

 ductive passages, both male and female. Ciliated epithelium 

 of spheroidal character is common in the invertebrate and 

 lower vertebrate animals, and is found in the ventricles of 

 the human brain. 



40. Secretion. It has been pointed out that epithelial 

 cells are in some instances devoted to secretion, and it will 

 be convenient in this place to explain the nature of that 

 function. Secretion is the preparation and separation of any 

 substance from, the economy; and the substance may either 

 pre-exist in the blood and be merely extracted therefrom, or 

 it may be a new substance elaborated by the secreting organ. 

 Thus, in the perspiration there is little which does not pre- 

 exist in the blood, while in the bile the most important in- 

 gredients not only are not constituents of the blood, but are 

 poisonous when absorbed into it. In probably all instances, 

 nucleated corpuscles of the description of epithelial cells are 

 agents in secretion. The water contained in some secretions 

 may escape from the blood in part by mere percolation, but 

 the solid constituents are selected or manufactured by the 

 secreting corpuscles or cells. 



A certain amount of serous or mucous secretion may 

 escape from, the general surface of serous or mucous cavities j 

 but for the production of special secretions there are glands 

 provided, which are organs composed essentially of recesses 

 opening off from epithelial surfaces and lined with secreting 

 cells, which obtain the material on which they act from a 

 copious supply of blood-vessels round about. The recesses 

 of which glands consist may be either tubules or saccules, 

 which may open singly on a free surface, constituting simple 

 glands, or may be gathered into groups, forming compound 

 glands, which, whether tubular or saccular in the secreting 

 part, pour their secretion into tubules converging to a 

 common duct. Compound sacculated glands have the 

 primitive saccules arranged in lobules, and these in larger 

 lobules ; and such glands are therefore termed lobulated. 



With respect to the mode of action of the secreting cells 



